Zen-Oh never viewed Zamasu as a threat to anything, he viewed Zamasu as a nuisance. Zen-Oh is, demonstrably, devoid of critical or rational thinking, and when presented with a problem that even wildly upsets him, he responds with absolute destruction. Have you forgotten that, in the past, Zen-Oh erased six entire universes in the blink of an eye because he was in an "unpleasant mood"?
Zen-Oh choosing to erase the entire world is not indicative of Zamasu being a threat to the entire world; it is indicative of Zen-Oh being an immature manchild who abuses his power at a moment's notice if his emotions sway him to do so.